Cavity resonator oscillator device



March 14, 1950 J. R. PIERCE 2,500,430

CAVITY RESONATOR OSCILLATOR DEVICE Filed July 28, 1944 2 Sheets-Sheet 1 FIG.

VOLTAGE AN Tl-NODE VOLMGE NODE INVENTOR By JRP/ERCE WWW A TTOPNEV March 14, 1950 J. R. PIERCE 2,500,430

CAVITY RESONATOR OSCILLATOR DEVICE Filed July 28, 1944 2 SheetsSheet 2 FIG. 2 3

IN [45 N TOR By JR PIERCE A TTORNEV QFFICE UNITED STATES CAVITY RESONATOR OSCILLATOR DEVICE John R. lierce, Millburn, N. 3., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 28, 1944, Serial No. 547,046

3 Claims. (Cl. SIS-39) This invention relates to oscillators and more Fig. 6 is an exploded view of the arrangement particularly to microwave oscillators of a type shown in Fig. 5. employing a resonator which is capable of os- Referring to the drawings, a magnetron is cillation in more than one mode, the respective shown at H! with a section of output transmisfrequencies of some of which modes may not orsion line it attached thereto, the line constitutdinarily be widely separated from each other. ing or representing a load circuit for the magne- An object of the invention is to suppress ostron. The latter is mounted between a pair of cillations of one or more of the modes while permagnetic pole-pieces i2 and !3. Attached to the mitting oscillations of a particular desired mode. pole-piece i2 is a tuning mechanism i l for ad- Another object of the invention is to make the lusting a tuning element inside the magnetron. frequencies of a lurality of the modes simulta- The magnetron H] comprises an outer cylinneously adjustable in order to maintain a subdrical shell l5, an inner conductor to (Fig. 2) and stantially constant frequency difference between a pair of end plates i7 and E8. The shell 55 may a desired mode and a mode which is to be supbe formed with a partial partition l9 dividing the pressed, at all settings of the frequency of the int ri r of th magn r n in a r s n in desired mode. chamber 25 and a cathode mounting chamber 2|.

The invention is illustrated as it may be applied The inner conductor I 6 may comprise a solid ferto a microwave oscillator of the magnetron type, romagnetic core member 2'2 (Fig. l) and a holalthough it is to be understood that the invention low conductive member 23. The partition 19 is not limited either to a microwave system or to 20 preferably contains a central cylindrical opena magnetron. The resonator of the oscillator is ing 24, the diameter of which may be equal to the shown as being a cavity resonator of the coaxial i er diameter of the member 23.

type. The resonator is illustrated as being con- The shell i5, end plate I1, partition is and nected through a coupling slot or dielectric wininner conductor l6 form the resonating chamber dow to an output transmission line. The cou- 2%, which is of the coaxial type with a gap 25 at pling means is so arranged and the transmission 0119 End Of the inner Conductor- A plurality of line is so oriented with respect to the resonator interleaving anode Segments 5 d 271 may e a that the transmission line is in effect coupled to tackled, respectively, 130 the inner Su o t e the resonator with respect both to the desired and element 255 d the inner Surface of the p to an undesired mode of oscillation of the reso- 24 in the partition I9. The arrangement of nator. To secure suppression of the undesired the anode Segments is indicated in Figs- 1, 4, 5 mode, while permitting the desired mode to be and 1 sh wing a lon itud nal s n. sustained, th coupling for Waves of the unde- 4 a cross-section, Fig. 5 a perspective view of the sired mode is made to be closer than the coup ing Segments With the Supporting surfaces in for waves of the desired mode. The relativel dash lines, and Fig. 6 an exploded view. A oath close coupling in the undesired mode of oscilla- Ode 23 of conventional form is Shown mounted on tion mean a heavy loading of t mode which a pedestal 29 which is in turn secured to the conresults in a suppression of oscillations. The rela- Vex side of a Cup-Shaped conductive member tively light loading of the desired mode may be A flange 9 of the member 3a is Supported adjusted to a value suitable for efficient producfrom the partltlon as by means of a mummy tion of sustained oscillations. It is to be underof insulating studs 3L The member 39 and studs stood that the invention is not limited by the type are Contained in the Cathode mounting cham of coupling nor by the kind of transmission line h A ferromagnetic core may be inserted employed but resides rather in means to provide m the f plate opposite f to materially closer coupling for one mode of osa??? Improve the magnetic clrcult of the cillations than for another whereby one mode is suppressed and the other sustained.

In the drawings, Fig. 1 is a view partly in longitudinal section and partly broken away, showing an embodiment of the invention in a magnetron oscillator;

The output transmission line H has an open end fitted over the curved outer surface of the shell l 5 as shown most clearly in Fig. 4. The line guide. The longitudinal axis of the guide preferably extends radially with respect to the resol 2 31 plan View of the Oscillator of 1; nator, and the transverse axes and the'sides of Fig. 3 1s a cross-sectional view showing one end t wave guide are t t a angle a. with the wall of the cavity resonator of the oscillator; 5 axis of the resonator, as indicated in Fig. 2. A Fig. 4 is a cross-sectional view showing the arslot or aperture is provided to permit the proparangement of the anode segments of the magnegation of electromagnetic waves out of the resotron and details of the output coupling; nant chamber 2i! into the transmission line H.

Fig. 5 is a perspective view of the assembly of The aperture may be filled with dielectric matei l is illustrated as a hollow rectangular pipe wave anode segments; and V rial in the form of a window 32 to serve as part of the evacuated envelop while allowing the waves to pass through the aperture without undue loss. The aperture is shown as rectangular in shape but it may have any other as desired, and when the aperture is to be filled with glass, for example, a circular or oval shape may be preferred.

An annular tuning element 33 is provided in the resonant chamber 29. The element 33 is shown as conductive ring but may be composed of dielectric material if desired. The element 33 preferably does not make electric contact with the surface of the chamber 20 and may be supported by a plurality of insulating rods 34 which pass out through holes in the end plate ii. The rods 3d may be rigidly connected to a ring 35 external to the chamber 20. Suitable means such as metallic bellows 36 may be provided between the ring 35 and the external surface of the end plate ll to prevent the entrance of air into the chamber 20 through the holes containing the rods 3d. The ring 35 may have, attached thereto or made integral therewith, an externally threaded sleeve member 3? slidably mounted upon the pole-piece l2. be rotatably mounted in mesh with the thread on the sleeve 3'! by suitable clamps 39 attached to the pole-piece l2 and provided with a lip 46 arranged to extend into a groove 4! in the body of the nut 38.

The end plate H may contain a radial groove 82, which might even be extended through the plate to form a slot and be filled with dielectric material. Alternatively, the partition it! may contain a radial slot or groove. The purpose of the slot or groove will be disclosed hereinafter.

In the operation of the device described, a steady magnetic field is applied by means of the pole-pieces i2 and i3, and the magnetron it, having been suitably evacuated, is supplied with heating currents for the cathode 28 and with a steady potential difierence between the cathode and the anode, all by means which are not shown but which are well known in the art and do not form any part of the present invention.

Oscillations with field patterns which are distinguished from each other only by having a difierent number of nodal or anti-nodal regions will be referred to hereinafter as oscillations of different order, whereas oscillations with field patterns diiiering more fundamentally will be referred to as oscillations of different mode. An oscillation of the first order may have one node and one anti-node, an oscillator of the second order two nodes and two anti-nodes, etc. This terminology is in accordance with common usage.

An advantage is to be gained by exciting the resonator to a higher order of oscillation rather than the first order, which latter is characterized by a single voltage node at the end plate i7 and a single voltage anti-node between the anode segments 26 and 27. For example, a second order oscillation may be used in which a second voltage node is established at a position indicated by a dot-dash line &3 (Fig. 1) near the anode segments and a second voltage anti-nodeis formed at a position between the end plate ll and the voltage node line 43 as indicated by'a dot-dash line it. As in the first order'oscillation of the same mode,

there is a voltage anti-node between the segments 26 and El. By using the higher order of oscillation, a condition is obtained in which most of the energy stored in the resonating system is stored in the distributed reactance of the coaxial resonator and very little in the lumped capacitance A captive knurled nut 38 may between the anode segments. As a result, the velocity of propagation of the waves is increased in the higher orders of oscillation as compared with the lowest order and this has the eliect of separating to a greater degree the frequencies of different modes of oscillation of any given order. An additional advantage of a higher order of oscillation is a greater stabilizing power in the resonator, reducing the frequency change or pulling attendant upon varying the impedence of the load circuit. The resonator, operating in the second order oscillation may be thought of as a distributed reactance or coaxial transmission line a little over a half wave-length long. The inductive reactance of the portion beyond the half wave-length is just sufficient to tune out the capacitive reactance of the anode segments.

The mode of oscillation just considered, in which the resonator acts as a coaxial transmission line, is characterized by the presence of longitudinal currents in the cylindrical conductive surfaces, that is, currents flowing parallel to the central axis of the resonator. The system. however, is capable of another mode of oscillation in which the resonator acts like a rectangular wave guide which has been rolled about the central axis to form an endless ring. Whereas in the coaxial resonator the phase of the standing wave does not vary with the angular position about the circumference of the resonator, the endless ring wave guide is capable of supporting oscillations in which the amplitude of the oscillations does vary according to the angle. The frequency separation of this mode from the desired mode is found to be good when the velocity of propagation in the endless ring wave guide is suificiently high.

Methods of causinga given oscillator to produce higher order oscillations are well-known in the art, and need not be described in explaining the present invention.

The purpose of setting the output transmission line i! at a suitable angle a with respect to the central axis of the resonator is, in accordance with the invention, to suppress the undesired modes of oscillation while permitting the desired mode to be sustained. When the angle a is made large, particularly exceeding 45 degrees and approaching 90 degrees, the coupling aperture couples much more strongly to the circumferential currents in the resonator walls than to the longitudinal cu:- rents therein. This effect will be evident from a consideration of the direction of current fiow in the transmission line H. The direction of the electric field lines in the wave guide is assumed to be parallel to the narrower sides of the rectangular guide, as in the usual practice. The accompanying electric currents flow in the broader sides of the guide and are longitudinal with respect to the guide (perpendicular to the plane of the paper in Fig. 2). The broader side of the wave guide approaches parallelism with the central axis of the resonator as the angle 0: approaches 90 degrees. Thecloser this approach, the more readily may the circumferential currents in the resonator flow into the. broader sides of the output transmission line. around the edges of the aperture as indicated by the arrows all and 46 in Fig. 4. The longitudinal currents in the resonator have a smaller and smaller component in the direction to flow through the aperture into the broader sides of the line, the nearer the angle on approaches 90 degrees. Hence the coupling to the undesired modes of oscillation is strengthened and the coupling to the desired mode is weakened by increasing the angle cc. Thus, the angle at may be chosen to eifect heavy loading of the undesired oscillations that will suppress the same, together with suitable or optimum loading for the desired oscillations.

The tuning element 39 constitutes a lumped capacitance between the shell l5 and the inner conductor E6, the amount or effectiveness of the capacitance depending upon the axial position of the element 33 with respect to a voltage antinode such as the one located at 44. The tuning element 33 is shown in the short wave position, that is, in a low voltage location where the capacitance introduced by the tuning element is relatively small. By means of the adjusting mechanism 14, the tuning element 33 may be moved closer to the voltage anti-node 44, that is, to a high voltage or long wave position where the capacitance eiiect of the tuning element is relatively great. Changing th position of the tuning element 33, has practically no tendency to introduce distortion in the electric field between the segments 28 and 21. In other words, the tuning leaves the angular symmetry of the pattern of the electromagnetic field unaffected. The tuning operation tunes the desired and undesired modes together and in the same direction, thus maintaining a substantially constant frequency separation between the modes. This latter effect will be evident when it is observed that thetwo modes of oscillation have voltage anti-nodes and voltage nodes in substantially the same position, as indicated by the lines 44 and 43.

The undesired mode under consideration in which the field intensity varies with the angle about the circumference of the resonator may be degenerate in the sense that there may be nothing which will serve to fix the nodes and anti-nodes at any particular angular location about the circumference. There is then likely to be a tendency for the nodes to progress around the circumference and assume an orientation in which there is relatively little coupling between the resonator and the wave guide. This possibility can be eliminated by breaking up the axial symmetry of the resonator in such a way as not to interfere with the desired mode. This effect can be achieved by means of a radial or longitudinal slot or a groove of appropriate depth in the resonator wall. Preferred orientations for such slots or grooves with respect to the wave guide output may be found. Such a radial groove 42 is illustrated in Fig. 3.

The ferromagnetic central conductor 22 may be coated or plated with a good conductor such as gold or silver, as may also the remaining conducting surfaces of the chamber 20, to reduce heat losses due to the surface currents.

The cup-shaped member 30 in the cathode supporting structure may be so shaped as to form a choke mounting. An indented central portion ll may cooperate with the surface of the opening 24 to provide a required amount of inductance to resonate the cathode-anode capacitance. The remainder of the convex surface 48 of the member 30 may cooperate with the opposite surface of the partition l9 to constitute a radial quarter wave transmission line section that is open circuited at the periphery and consequently presents a low impedance as viewed from the central axis. The insulating supports 3! are preferably placed as shown, in the high impedance open end of the radial transmission line. The choke mounting serves to prevent the escape of high frequency energy from the resonating chamber 20 into the cathode mounting chamber 2 I.

What is claimed is:

1. An oscillator comprising a cylindrical cavity resonator, an anode and a cathode coaxial with and mounted within said cavity'resonator, a hollow pipe wave guide of rectangular cross-section mounted upon the external surface of a portion of the wall of said cavity resonator and having its longitudinal axis extending radially with respect to said cavity resonator, the side walls of said hollow pipe wave guide being oblique to the longitudinal axis of said cavity resonator, and a dielectric wave permeable window in the said wall between said cavity resonator and said hollow pipe Wave guide.

2. A magnetron comprising a pair of magnetic pole-pieces axially aligned, a cylindrical cavity resonator mounted between said pole-pieces and coaxial therewith, an axially extending inner conductor supported from one end of said cavity resonator and coaxial therewith, a transverse apertured conductive diaphragm mounted inside said cavity resonator adjacent to the free end of said inner conductor, a hollow cylindrical array of interdigitated anode elements alternate ones of which are conductively attached to said inner conductor and the remainder of which are conductively attached to said apertured diaphragm around the edge of the aperture therein, a choke mounting in the space within said cavity resonator on the side of said apertured diaphragm opposite from said inner conductor, and a cathode attached to said choke mounting and projecting through the aperture in said diaphragm into the space surrounded by said array of anode elements.

3. A magnetron comprising a pair of magnetic pole-pieces axially aligned, a cylindrical cavity resonator mounted between said pole-pieces and coaxial therewith, an axially extending inner conductor supported from one end of said cavity resonator and coaxial therewith, a transverse apertured conductive diaphragm mounted inside ,said cavity resonator adjacent to the free end of said inner conductor, a hollow cylindrical array REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,153,728 Southworth Apr. 11, 1939 2,241,119 Dallenbach May 6, 1941 2,247,077 Blewett et al. June 24, 1941 2,250,698 Berline July 29, 1941 2,272,211 Kohler Feb. 10, 1942 2,300,052 Lindenblad Oct. 27,1942 2,396,044 Fox Mar. 5, 1946 2,415,242 Hershberger Feb. 4, 1947 2,425,345 Ring Aug. 12, 1947 

